Kimi no Tame nara Nandodemo

Yuugo, the protagonist, is deeply in love with Shiina-chan and has witnessed her death countless times. His abnormal love for her has granted him a "gun" with the power to transcend space and time. However, every reset comes at a cruel cost: causing her memories to fade one by one. Even so, the burning flame of his love never goes out. To save Shiina-chan from her untimely demise, and until Yuugo can reach the one irrefutable "proof of love"—a kiss—he won't stop pulling the trigger.

(Source: Akita Shoten, translated)